Known optical discs are information storage media with a sector structure. Optical discs are roughly classifiable into the following three types according to property. A first type is read-only optical discs in which data is stored by the unevenness of their surface and to which the user is not allowed to add any new data. A second type is write-once optical discs, which include a recording film of an organic dye, for example, and on which data can be written only once. And a third type is rewritable optical discs, which include a recording film of a phase change material, for example, and on which data can be written a number of times (i.e., rewritten).
As audiovisual data including audio data and video data (which will be collectively referred to herein as “AV data”) is recently broadcast or downloaded in the digital format more and more often, there is a growing demand for optical discs with even higher storage densities and even bigger storage capacities. To increase the storage capacity, it is an effective measure to take to provide multiple information layers for a single disc. For example, in a DVD as a read-only optical disc, the single optical disc is provided with two information layers, thereby almost doubling the storage capacity compared to an optical disc with only one information layer. Furthermore, a Blu-ray Disc (BD) that was developed just recently has storage density per information layer that is approximately five times as high as that of a DVD, thereby realizing a huge storage capacity of 50 GB by the two information layers combined. And to further increase the storage capacity, discs with four or even six information layers are now under research and development.
As the storage capacities have been increased in this manner by providing an increasing number of information layers for a single disc, more and more new methods for managing optical disc areas have been proposed. For example, Patent Document No. 1 discloses an optical disc area management method for reading a defective area more quickly even in a situation where the defect list area of the reference layer (i.e., the first information layer) of a multilayer optical disc has become no longer available because that layer is already full or has a defect itself. Meanwhile, Patent Document No. 2 discloses an optical disc area management method for writing data in real time on a BD-R, which is a write-once medium (i.e., a storage medium on which data can be written only once), and for reading the stored data even more accurately.                Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 2003-323769        Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 2006-24287        